This is about the moment when you’re aiming UP and you need to switch from left to right in the same position. I am suggesting an automatic turnaround; when you aim upwards in one direction the aim should rotate and face the opposite direction automatically. Automatic Turnaround. Maybe people have not considered this a problem, but i have, and i believe it could be improved.
The weapon will spin at constant speed even after switching from left to right in the turnaround, without stopping, and you could calculate your shooting easier and smoother. I don’t think this overpowers the controls, because the player jumping on you will still have to time their counter block, to deflect the bullets either way. In this scenario, more flexibility for a shooter makes no difference for the attacking enemy, in my opinion. If you think otherwise, please explain.

You no longer have to press 4 keys in total to do this entire thing [ Presently this happens: Pressing D, then UP↑, until the aiming freezes in the upwards position, traveling a circle-quarter, you can't aim any further -see the gif- , in order to do that you have to press either RIGHT→, or LEFT←, to help the little man turn around, and continue its semicircular aim]. With this suggestion applied, you don’t have to press either RIGHT→, or LEFT← because UP↑ already does that, by automatically turning the character around, doing an uninterrupted semi-circle. It’s similar to the portal controls, when you go into a portal, you have to hold the same key to go left even if you're pressing right, ‘til the moment you switch to the left arrow button.
It’s not as complicated as it sounds it’s very simple and smooth if you think about it and try to imagine. Presently you have to press either left or right arrow keys to switch to the other side, then in order to aim again towards an enemy you have to move your fingers fast to press the “down” arrow key. Personally this is very fiddly, working with one hand.

This is not only my issue. I’ve seen people in multiple instances where they failed to switch their aim and defend themselves from a falling enemy ready to attack. Presently this way of switching from right to left while aiming upwards feels very unnatural, and I think this is a good suggestion to help the upward aim run smoother. This is a very simple change and the situations which require this movement don't happen so often, but when they do, they tend to be catastrophic for your health bar.

If people won’t like this slight change, then we should have it as an ON/OFF option available in the control settings. If you think this is not necessary or will not help at all, or maybe you think this could be done better, please feel free to explain why or how.

It does look like a good feature, but I think it will just make guns more powerful.
First situation that comes to mind where it can be really powerful, is when the gunner is above someone who doesn't have one themselves. What that person can do, is stay under him and constantly switch between both sides of the gunner, so the gunner will have a hard time aiming. And if that person is able to get close too, he has a chance atleast. And even after all of that, he has to melee the gunner, but atleast there will be a fight.
This is a way to safely get closer to someone who has a gun if you only have melee, it's almost like a blindspot that you can take advantage of. Removing that would remove another way to get close to a gunner, which I think is unfair. (Gun is more favoured than Melee already.)
But you surely have time to do something else, like maybe even aim there yourself if you're quick enough, or quickly unequip (Q) and block his swing, or run out of range.